Temperature compensated moisture meter having bridge zero varying potentiometer and ganged rheostats



Feb. 24, 1970 TEMPERATURE coMPENsATED MoIs'ruR-E METER HAVING BRIDGE zERo v VARYING POTENTIOMETER AND GANGED RHEOSTATS Original Filed Dec. 16. 1964 2 Sheets-Sheet `J. 4

EUR W -INVENTOR Lloyd V. Fagan Jr.

Feb. 24, 1970 v. FEGAN, JR 3,497,803

TEMPERATURE COMPENSATED MOISTURE METER HAVING BRIDGE ZERO VARYING'POTENTIOMETER AND GANGED RHEOSTATS Original Filed Dec. 16, 1964 2 Sheets-Sheet 2 1-- i w m Lloyd M Fagan Jr.

United States Patent O 3,497,803 TEMPERATURE COMPENSATED MOISTURE METER HAVHNG BRIDGE ZERO VARY- ING IDOTENTIOMETER AND GANGED RHEOSTATS Lloyd V. Fagan, Jr., Lebanon, Pa., assignor to Bethlehem Steel Corporation, a corporation of Delaware Original application Dec. 16, 1964, Ser. No. 418,814, now Patent No. 3,344,921. Divided and this application June 16, 1967, Ser. No. 646,710

Int. Cl. Gtllr 27/ 02 US. Cl. 320i- 65 1 Claim ABSTRACT OF THE DISCLOSURE Filter vacuum is controlled to produce cake having moisture content less than that desired in final product. Cake level in filter bin is maintained constant by continuously varying lter speed. Spaced probes are partially buried in bed of cake discharged from bin to conveyor belt. Resistance of bed of cake between probes appears as one resistance in Wheatstone bridge moisture meter which continuously controls addition of Water to =bed of cake to bring moisture content to desired value. Slurry temperature in lter is continuously taken and temperature correction is continuously introduced into moisture meter.

CROSS-REFERENCE TO RELATED APPLICATION This application is a division of my copending application Ser. No. 418,814, tiled Dec. 16, 1964, and titled, Method and Apparatus for Measurement and Control of Moisture, now Patent No. 3,344,921 which issued on Oct. 3, 1967.

BRIEF SUMMARY OF THE INVENTION This invention relates broadly to a moisture meter. More specifically, this invention relates to electrical apparatus of the Wheatstone bridge type to continuously measure moisture in a moving bed of material by measuring the resistivity (or conductivity) of the bed between a pair of spaced probes or electrodes, with means to measure the temperature of the material and to introduce a temperature correction into the moisture measurement.

The present invention will best be understood by description in a specific environment or application.

Because of the depletion of high grade iron ores in this country, the iron ore mining industry has turned its attention to the beneciation or upgrading of the more plentiful low grade iron ores. Through such beneiiciation or upgrading, the industry is able to provide a high grade uniform raw material for subsequent use in the blast furnace. Before this high grade raw material or iron or concentrate can be used in the blast furnace, it must be aggolmerated into pellets in a balling drum or balling cone, the green pellets issuing from the drum or cone then passing to a pelletizing furnace, which may for instance be of the shaft type or of the travelling grate type, wherein the green pellets are heat-hardened and are then ready for shipment to and use in the blast furnace. The desirable properties of the pellets, i.e., strength, uniformity of size and resistance to deformation, are directly related to the moisture content of the iron ore concentrate before being formed into green pellets. Moreover, the successful operation of the drum or cone in forming the iron ore concentrate into green pellets also depends upon the moisture content of the iron ore concentrate feed. Thus, if the iron ore concentrate is too wet or too dry, it will slide in the drum or cone and will not agglomerate into pellets. In fact, the moisture content of the iron `ore conice centrate is quite critical and, for instance, in the balling cone, the moisture content of the iron ore concentrate feed should prefera-bly be held to within i0.2% of the required value for a satisfactory product.

lt is essential, therefore, that the moisture content of the iron ore concentrate being agglomerated into green pellets be accurately measured and closely controlled and as the pelletizing process is continuous rather than batchtype, that the measurement and control of moisture content be continuous, in order that a uniformly satisfactory product may be produced.

The conductivity of the iron ore concentrate varies with its moisture content among other things; that is to say, the higher the moisture content of the iron ore concentrate, the greater will be the capacity of the iron ore concentrate to conduct electricity. The present invention utilizes conductivity measurements (or the corollary function, resistance) to monitor the moisture content of the iron ore concentrate, and employs a pair of horizontally spaced probes immersed in the moving bed of iron ore concentrate passing to the balling drum or cone, through which probes an electrical current is passed, the resistance of the iron ore concentrate to the passage of the electrical current between the probes being measured by the degree of unbalance of a Wheatstone bridge and interpreted as moisture content.

The use of probes per se to measure electrical resistance, and hence moisture content, in a moving bed of particulate material is not new. Infact, it is well known that the moisture content of bulk materials such as iron ore concentrates, foundry sand, coal, etc., can be determined through the measurement of the electrical resistance of these substances, and this method is today being used in industry with conventional controlling systems. However, none of these systems incorporates means to continuously measure the temperature of the material being examined for moisture content and for automatically and continuously introducing a temperature correction into the Wheatstone bridge circuit. Under some circumstances, electrical resistance can vary ap-preciably with temperature variations alone, and it has been often found that the continuous introduction of a temperature correction is often essential to the successful operation of a system intended to continuously and accurately measure and closely coI1- trol, particularly within the close tolerance (i0.2%) hereinbefore mentioned, the moisture content of a moving bed of normally conductive particulate material such as rron ore concentrate.

One of the objects of this invention is to provide an improved moisture meter for continuously and accurately measuring the moisture content of a moving bed of particulate conductive material such as iron ore concentrate.

Another object of this invention is to provide an improved moisture meter incorporating means to continuously and automatically correct for variations in the temperature of the moving bed of particulate conductive material.

Other and further objects of this invention will become apparent during the course of the following description and by reference to the drawing and to the appended claim.

Briefly, the present invention in the preferred embodiment comprises a Wheatstone bridge, one of the resistances thereof appearing across a pair of spaced probes at least partially buried in a moving bed of material, through which probes an alternating current is passed. The temperature of the material is continuously taken by means of a thermocouple, and the thermocouple output continuously operates a potentiometer which continuously adjusts certain of the resistances in the Wheatstone bridge, thereby to continuously and automatically introaereos duce a temperature correction into the measurement of moisture content.

BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings, in which like numerals represent like parts in the several views:

-FIGURE 1 represents a flow diagram of a system for contr-olling the moisture content of a moving bed of particulate conductive material, and employing the present invention.

FIGURE 2 represents the circuit diagram of the present invention, together with certain portions of related apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will be described in its application to a particular system for continuously controlling the moisture content of a moving bed of particulate conductive material. It will, however, Vbe understood that the present invention is capable of use with other moisture control systems and the like.

In the moisture controlling system shown in FIGURE 1, filter 1 is a continuous vacuum rotary filter as of the Ameri-can type (Chemical lEngineers Handbook, third edition, page 981), having filter discs 2 mounted to hollow shaft 3, and vacuum chest 4 communicating through the hollow shaft 3 with the interiors of the filter discs 2. Iron ore slurry 5 from the benefication operation is introduced into the filter feed tank 6 through a suitable conduit (not shown). Hollow shaft .3 and filter discs 2 connected thereto are rotated by pneumatically controlled infinitely variable speed drive 7 as of the Reeves type.

Bin 8 is mounted below filter 1, is open at its bottom, and overlies belt conveyor 9. Bin 8 receives filter cake 10 (f e., iron ore concentrate) removed from filter discs 2, and belt conveyor 9 removes said filter cake 10 through opening 11 in one vertical wall of bin 8 whereby a bed 12 of uniform height is provided on belt conveyor 9. This bed 12 is not necessarily of uniform bulk density and, as the measurement of moisture content in bed 12 as hereinafter described is based on conductivity determinations, and as the iron ore concentrate is normally conductive, variations in bulk density in bed 12 may introduce considerable error into the measurement of the moisture content thereof. Thus, it is desired that bed 12 have substantially uniform and constant bulk density. It has been found that substantially uniform and constant bulk density may be realized by maintaining a generally triangular pile of filter cake 10 in bin 8 behind opening 11, which pile has a vertical leg adjacent the forward wall of bin 8 (i.e., the wall with opening 11 therein) of substantially constant height, and sloping downwardly at a substantially constant angle toward the rear of bin 8, and permitting belt conveyor 9 to remove filter cake 10 from bin 8 uniformly along the base of the triangular pile of filter cake 1l) shown diagrammatically in FIGURE 1. To this end, means are provided to regulate the speed of rotation of filter discs 2 and, therefore, the production of filter cake 1t). Such means may comprise an electrical circuit including the normally conductive filtered cake 10, whereby a probe is positioned at the desired height in bin 8 at the desired distance behind the front wall thereof, contacting the surface of the triangular pile of filter cake 16 to complete an electrical circuit and reduce the speed of rotation of filter discs 2, and, hence, the rate of production of filter cake 10; when the triangular pile of filter cake 10 falls below the desired height, contact between the probe and the normally conductive filter cake 10 is broken and means are thereby actuated to increase the speed of rotation of filter discs 2 and, hence, the rate of production of filter cake 10. In the system illustrated, free-swinging, nonrigid electrically conductivl:V insulated cable 13, with electrically ccnductve weight or beh 14 Secured to the end thereof, is suspended in bin 8 by means of insulated support 15. Bin 8 is indicated as electrically grounded by line 16. Filter speed control means represented diagrammatically by block 17 in FIGURE 1 communicates electrically with cable 13 and is also electrically grounded, it being understood that, in place of actual grounding connections for bin 8 and control means 17, another line may run directly from bin 8 to control means 17. When the pile of normally conductive filter cake 10 rises in bin 8, bob 14 contacts the surface thereof completing an electrical circuit including control means 17, and the latter produces a pneumatic signal transmitted to pneulmatically controlled infinitely variable speed drive 7 through line 18 to reduce the speed of rotation of filter discs 2 and hence to reduce the rate of production of filter cake 10. Thereafter, when the pile of normally conductive filter cake 11i falls in bin 8, bob 14 leaves the surface thereof breaking the electrical circuit including control means 17, and the latter produces a pneumatic signal transmitted to pneumatically controlled infinitely variable speed drive 7 through lin-e 18 to increase the speed of rotation of filter discs 2 and hence to increase the rate of production of filter cake 10. Ideally, bob 14 is constantly making and breaking contact with the surface of filter cake 10. The angle of repose of filter cake 10 is a constant and, when the position of bob 14 above belt conveyor 9 and behind the front wall of bin 8 is determined, it will be apparent that the height of the vertical leg of the triangular pile of filter cake 10` is also determined. The height of bob 14 above belt conveyor 9 should not be too great, as the filter cake -10 may feed belt conveyor 9 only from the front of the triangular pile of filter cake 10.

Downstream of bin 8, t-wo probes 19, horizontally spaced at a fixed distance from each other, are partially immersed in bed 12 and, with apparatus hereinafter described, are adapted to continuously measure the resistance, and hence the moisture content, of the iron ore concentrate moving between the said probes 19. Downstream of probes 19, a drip pipe 2t) positioned across moving bed 12 adds water thereto as needed to adjust the moisture content of the iron ore concentrate to its required value.

The bed 12 of iron ore concentrate on belt conveyor 9, after having its moisture content measured and adjusted, is discharged to an inclined belt conveyor 21, forming a bed 22 thereon. A binder, such as bentonite, is fed from hopper 23 to bed 22 at the desired rate by means of conventional rotary feeder 24 and feed pipe 25. Downstream of feed pipe 25', mixer 26 thoroughly mixes the iron ore concentrate, the binder and the water previously added. The mix is then fed into pelletizng apparatus such as a balling cone 27. As the balling cone 27 rotates, the mix agglomerates into green pellets 28 which are discharged from the balling cone 27 and are then passed to a pelletizing furnace (not shown) wherein they are heathardened for subsequent use in the blast furnace.

The moisture meter of the present invention, indicated diagrammatically in FIGURE 1 by block 29 is essentially a device for continuously measuring the resistance of the bed 12 of iron ore concentrate passing between probes 19. A low voltage alternating current is passed between probes 19 through the iron ore concentrate and an output appears across a Wheatstone bridge in moisture meter 29 inversely proportional to the resistance of the said iron ore concentrate between probes 19. Moisture meter 29 is connected to probes 19 through leads 3). Variations in temperature will affect the measurement of resistance of bed 12 of iron ore concentrate, and may introduce serious errors in the determination of the moisture content thereof.

It will be apparent that variations in temperature of the iron ore concentrate in bed 12 will follow variations in the temperature of the iron orc slurry 5 in filter feed tank 6 of filter 1. To introduce a temperature correction into the measurement of resistance, and hence moisture content, of the iron ore concentrate in bed 12, thermocouple 31, indicated diagrammatically in FIGURE 1, is conveniently immersed in iron ore slurry 5 in filter feed tank 6. Leads 32 connect thermocouple 31 to a temperature compensating control unit, indicated diagrammatically by block 33 in FIGURE 1. Temperature compensating control unit 33 includes amplifier 34 amplifying the small electrical output (perhaps between -5 millivolts) of thermocouple 31, a relay 35 connected to power lines (not shown) and operated by the output of amplifier 34 to actuate motor 36 and potentiometer 37 preferably of the pot type having wiper contact 38 secured to and rotated by shaft 39 of motor 36 and resistance 40. Leads 41 and 42 connected to the ends of resistance 4t), and lead 43 connected to wiper contact 38, are connected to one of the legs of the Wheatstone bridge in moisture meter 29 as shown in greater detail in FIGURE 2.

A voltage regulator, indicated diagrammatically by block 44 in FIGURE 1, and which is essentially an isolation transformer with an output voltage that remains constant despite uctuations in input voltage, is interposed between moisture meter 29 and 11S-Volt A.C. power lines, whereby moisture meter 29 is powered by a stable source of alternating current and is isolated from surges which may appear in the 11S-volt A.C. power supply due, for instance, to start up or stopping of heavy electrical machinery connected to the same source of current.

FIGURE 2 shows a circuit diagram of voltage regulator 44, the preferred form of moisture meter 29, and potentiometer 37, and will be described further on in this specification. The output of moisture meter 29, which may for example range between 0-30 millivolts, is fed through leads 45 to a moisture recorder-controller, indicated diagrammatically in FIGURE 1 by block 46. Moisture recorder-controller 46 is a standard pneumatic recordercontroller device having an electrical input and a pneumatic output, preferably of the type known as proportional, producing a pneumatic signal or output, in response to the electrical input of moisture meter 29, varying, for example, between 6-10 p.s.i. in the normal 0 15 p.s.i. instrument air range. The pneumatic output of moisture recorder-controller 46 is fed through line 47 to the set-point-adjusting input of a standard ow controller, represented diagrammatically by block 48 in FIGURE 1. An orifice meter, represented diagrammatically by block 49 in FIGURE l, is installed in water line 51 which leads to drip pipe 20, and pressure taps 54 upstream and downstream of the orifice plate in orifice meter 49 communicate with the differential pressure inputs of said ow controller 48, thereby feeding back into said ow controller 48 a signal indicative of the actual fiow rate through orifice meter 49. The pneumatic output of fiow controller 48 is fed through line 55 to air-operated water control valve 50 in water line 51. A pressure regulator 52 may be placed in water line 51 before orifice meter 49, and a flow meter 53 is placed in water line 51 between water control valve 50 and drip pipe 2t) for calibration purposes.

Continuing the description of the system shown in FIG- URE 1, a vacuum recorder-controller is indicated diagrammatically by block 57. Vacuum recorder-controller 57 is a standard pneumatic recorder-controller device preferably of the type known as proportional plus reset having a pneumatic input to adjust the set point, a process or vacuum input for the proportional control function, and a pneumatic output. Vacuum receiver or tank 58 is connected through line 59 with vacuum chest 4 of filter 1. Line 60 communicates between vacuum receiver 58 and the vacuum input for the proportional control function of vacuum recorder-controller 57. Manual load station 61 is essentially a manually operated air pressure regulating valve, served by l p.s.i. instrument air line 62. The output of manual load station 61 is connected through line 63 with the pneumatic input to control the set point of Vacuum recorder-controller 57. The pneumatic output of vacuum recorder-controller 57 is fed through line 64 to air-operated vacuum control valve 65 in vacuum line 66 leading to vacuum receiver 58. The intake or suction of vacuum pump 67 is connected to Vacuum line 66 and normally open block valves 68 are placed in vacuum line 66 on both sides of vacuum control valve 65, as shown in FIGURE 1. The position of vacuum control valve 65, which may for example be of the buttery type, is regulated by the pneumatic output of vacuum recorder-controller 57 and it will be apparent that with the foregoing arrangement the degree of vacuum in vacuum chest 4 can be controlled and regulated by manual load station 61 and line 60 to regulate the extent of dewatering of the filter cake on filter discs 2. Thus, the moisture content of filter cake 1t) entering bin 8 is subject to control. Vacuum receiver 58 provides vacuum capacity and acts as a sort of tiywheel to prevent sudden and wide surges in the degree of vacuum in vacuum chest 4 and thereby to prevent similar excursions in the moisture content of the filter cake or filter discs 2. In accordance with accepted practice, line 69 with normally closed valve 70 therein bypasses vacuum control valve 65, and in an emergency or when vacuum control valve is to be removed for repairs, block valves 68 can be closed and valve opened to permit continued vacuum filtering of the iron ore slurry.

It will of course be understood that moisture recordercontroller 46 and vacuum recorder-controller 57 have automatic-to-manual bypass panels by means of which the penumatic output thereof can be adjusted manually for calibration or for control purposes in case of emergency.

The operation of the system shown in FIGURE 1 will now be described.

The set point of vacuum recorder-controller 57 is adjusted by manual load station 61 to maintain such a degree of vacuum in vacuum chest 4 of filter 1 that the moisture content of filter cake 10 in bin 8 is only slightly below the required value. It will be appreciated by those familiar with this art that, as the filtering operating continues, the pores in the filter discs blind, and that a greater degree of vacuum will be required as time progresses to produce a slightly dry filter cake 1t) of the same moisture content and, consequently, the set point of vacuum recordercontroller 57 will require shifting with the passage of time. All the while, the speed of rotation of filter discs 2 is controlled by filter speed control means 17 to maintain a constant quantity of filter cake 1t) in bin 8. Belt conveyor 9 carries filter cake 10 through opening 11 in bin 8 and past probes 19 which, through leads 30, introduce a low Voltage alternating current through bed 12 of iron ore concentrate passing between the said probes 19. The resistance of the bed 12 of iron ore concentrate passing between probes 19 is continuously impressed on the Wheatstone bridge of moisture meter 29, as hereinafter described. At the same time, the output of thermocouple 31 actuates temperature compensating control unit 33 to introduce a temperature correction into the Wheatstone bridge of moisture meter 29. The output of moisture meter 29, which is a function of the resistance of bed 12 between probes 19 and the temperature of the slurry 5 in feed tank 6, is fed to moisture recorder-controller 46, which, in turn controls water control valve 50 in water line 51. The set point of moisture recorder-controller 46 is adjusted so that, normally, water control valve 50 permits a slight amount of water to be continuously added to bed 12 by means of drip pipe 20. Under ideal equilibrium conditions, this amount of water may be .3-.5 gallons per minute when the system is handling 30 tons per hour of iron ore concentrate. To insure a linear response of water control valve 50, to the signal from moisture recorder-controller 46, and to insure an accurate delivery of water through drip pipe 20 despite back pressures which might vary, due for instance to the plugging of holes in the drip pipe or for other reasons, flow controller 48 is interposed between moisture recorder-controller lo and water control valve Si), and orifice meter 019 in the water line 51 feeds back to flow controller 48 information con- Corning the actual water flow rate in the said water line 51.

It will be apparent that, by establishing a slight fiow of water from drip pipe 2f) at equilibrium, a positive or negative correction to the moisture content of bed 12 can always be made, by increasing or decreasing the flow rate from drip pipe 2f).

Closed-loop control (ie, the circuit from probes 19 back to vacuum chest 4 of filter 1) provides a coarse delayed adjustment of moisture content in the filter cake 16 and, standing alone, would permit slugs of iron ore concentrate of incorrect moisture content to pass to balling cone 27. Feed forward control (i.e., the circuit from probes 19 forward to drip pipe 20) provides fine and rapid adjustment of moisture content in bed 12 but is incapable of accurately correcting large deviations of moisture content from the required value. By combining closed-loop control with feed forward control, an extremely accurate (within 10.1%) and reliable adjustment of moisture content is made, and there is never a slug of off material passing to the balling cone 27.

FIGURE 2 shows the circuit diagram of the moisture meter 29, together with pertinent portions of related equipment.

Voltage regulator 44 (bounded by phantom lines) is seen as comprising isolation transformer S and receptacle 81. Plugk S2 of moisture meter 29 is inserted in receptacle 81, and leads 83 connect to the primary winding of a variable transformer 84. Leads S connect the secondary winding of variable transformer 84 to branches 86 and 87 of a Wheatstone bridge. Branch 86 includes fixed resistance 88, adjustable rheostats 89 and 90, and potentiometer 37 (bounded by phantom lines) of temperature compensating control unit 33. Leads 41 and 42 connect between the ends of resistance 4f) of potentiometer 37 and ganged wiper contacts 91 and 92 of adjustable rehostats 89 and 90, respectively, whereby calibration can be effected without changing the total resistance of branch 86 of the Wheatstone bridge, it being apparent that, during calibration, a decrease or increase in the value of either resistance 89 or 9() results in a corresponding increase or decrease, respectively, in the value of the other resistance 9o or 89. The other branch 87 of the Wheatstone bridge includes resistance 93 and the resistance of that portion of bed 12 of iron ore concentrate between probes 19, leads 30 from said probes 19 tying in to branch 87 as shown. A full-wave rectifier 94 is interposed between branches 86 and 87, being connected at one end to lead 43 of the potentiometer 37 of temperature compensating control unit 33 in branch 86, and being connected at the other end through lead 95 to branch 87 between resistance 93 and that portion of bed 12 between probes 19. The rectified output of fullwave rectier 94 is fed through leads 96 and 97 to an adjustable filter and delay circuit 98 comprising capacitors 99, resistance 161i and ganged multi-position switches 101. The output of moisture meter 29 is then fed through lines 45 to the moisture recorder-controller 46 for the system shown in FIGURE l, shunt 162 being provided as is known in the art to permit a maximum output voltage range of 0-30 millivolts.

By means of variable transformer 84, the sensitivity and span of moisture meter 29 can be adjusted.

Potentiometer 3'7 of temperature compensating control unit 33 introduces a temperature correction into the Wheatstone bridge to provide greater accuracy in the measurement of moisture in the bed 12 of iron ore concentrate.

The resistance of bed 12 will usually fiuctuate over short intervals of time, due to inefficient mixing of filter @also l() ,in bin 8 and, in continuous moisture measurement and control, the short-interval moisture variations, which may for example be 5-30 seconds apart, should be suppressed or eliminated. Moreover, there is an interval of time elapsing between the measurement of the moisture content of a slug of iron ore concentrate in bed 12 at the probe station and the arrival of this measured slug at the drip pipe station for moisture correction. Filter and delay circuit 98 permits the selection of various combinations of capacitors 99 to provide the desired degree of damping in short-term moisture signal variations as well as to delay the moisture signal sufiiciently to permit the measured slug of iron ore concentrate to pass from the probe station to the drip pipe station.

The operation of moisture meter 29 is believed to be apparent from the foregoing description. It should be noted that, if the moisture meter passed a D.C. current between probes 19, error would be introduced into the moisture measurement because of electrode polarization. The use of A.C. current with the present moisture meter eliminates this source of error.

By means of the foregoing apparatus and method, the moisture content of a moving bed of particulate material, such as iron ore concentrate passing from a filtering operation to an agglomerating operation, can be measured and controlled, reliably and, accurately.

I claim:

1. Moisture measurement apparatus to electrically measure the moisture content in a bed of material and produce a tempeature-corrected electrical signal indicative of the amount of moisture therein, said apparatus comprrsmg:

(a) a Wheatstone bridge having a first branch and a second branch,

(b) said first branch compising a first resistance means and a second resistance,

(c) said lfirst resistance means comprising a pair of spaced electrically conductive probes to be at least partially buried in a bed of material having variable electrical resistance properties and whose moisture content is to be measured electrically, the resistance across said probes being the only variable resistance in the first branch,

(d) said second branch comprising a third resistance means and a fourth resistance,

(e) said third resistance means comprising:

(l) a potentiometer having a sliding Contact actuated by temperature measurement means to temperature-correct bridge output, and

(2) first and second rheostats ganged and circuited to provide a variable reference point with respect to the potentiometer setting,

(3) whereby varying the setting of either the potentiometer or the rheostats only offsets bridge zero but not bridge calibration,

(f) temperature measurement means responsive to the temperature of said material associated with the probes for varying the setting of said potentiometer,

(g) means to impress an alternating electric current across said first and second branches of said Wheatstone bridge,

(h) means to detect an output voltage between the potentiometer sliding contact of the second branch and that portion of the first branch between the first resistance'means and the second resistance,

(i) said output voltage corresponding to the temperature-corrected electrically measured moisture content of said bed of material.

References Cited UNITED STATES PATENTS EDWARD E. KUBASIEWICZ, Primary Examiner 

